It is well known to use the combination of a radiation source, such as Cesium137 and an elongated radiation detector as a device for measuring the level of material, such as in a tank, that is situated between the radiation source and radiation detector. Such devices are particularly useful when the material being measured or the environment in which it is located are particularly caustic, dangerous, or otherwise not amenable to traditional level measurement devices.
Early continuous level detection devices used an ion chamber detector. For example, the ion chamber could be a three to six inch (7.5–15 cm) diameter tube up to 20 feet (6 meters) long filled with inert gas pressurized to several atmospheres. A small bias voltage is applied to a large electrode inserted down the center of the ion chamber. As gamma energy strikes the chamber, a very small signal (measured in picoamperes) is detected as the inert gas is ionized. This current, which is proportional to the amount of gamma radiation received by the detector, is amplified and transmitted as the level measurement signal.
Alternatively, elongated scintillation detector “crystals” have been used. Such devices are many times more sensitive than ion chambers and are also considerably more expensive. This added expense is often acceptable because it allows the use of either a smaller radiation source size or to obtain a more sensitive gauge. When gamma energy hits the scintillator material, it is converted into invisible or UV flashes comprised of light photons (particles of light). These photons increase in number as the intensity of gamma radiation increases. The photons travel through the scintillator medium to a photomultiplier tube, which converts the light photons into an electrical signal. The output is directly proportional to the gamma energy that is striking the scintillator.
Both ion chamber detectors and scintillation counter detectors have the disadvantage of being quite rigid in structure. In some applications, such as extending the detector vertically around a horizontally-oriented tank or along the length of a tank where the shape of the tank or obstructions which are on or part of the tank limit or prevent the use of such rigid detectors. There is a need for a scintillation counter detector that is flexible so that it may be adapted in the field to bend around such obstacles.
Fiber optic cables made of many individually clad strands of scintillator material have been presented as a solution to this problem. An example of this is shown in U.S. Pat. No. 6,198,103. The required individual cladding of these fibers, however, makes such a solution undesirably costly. Another example of a flexible scintillation crystal detector is shown in our U.S. Pat. No. 6,563,120, issued May 13, 2003. In these detector systems, the radiation source is positioned outside the container, the scintillation detector is positioned outside the container, and the product having its level being detected is within the chamber, as illustrated in FIG. 1 of the above-noted U.S. Pat. No. 6,198,103. The quantity of radiation received by the scintillation detector is an indication of the level of the product within the container.